1. Field of the Invention
This invention relates to a control system for hybrid vehicles having an internal combustion engine and a traction motor as prime movers.
2. Prior Art
Conventionally, a hybrid vehicle having an internal combustion engine (hereinafter simply referred to as "the engine") and a traction motor as prime movers is widely known, and a control system for controlling the prime movers of such a hybrid vehicle has already been proposed e.g. by Japanese Patent Laid-Open Patent Publication (Kokai) No. 5-229351.
The proposed control system determines the optimum torque at which the maximum engine efficiency is attained, in dependence on traveling conditions of the vehicle, and at the same time detects actual torque generated by the engine for actually driving the vehicle. Then, the control system determines or selects demanded or required torque from the optimum torque and the actual torque. The assistance of the traction motor to the engine is carried out based on the demanded torque when the vehicle is in a suitable condition, e.g. during acceleration of the engine.
However, the proposed control system suffers from the following inconvenience in respect of the efficiency of the engine:
For example, when the engine is operated with the air-fuel ratio of a mixture supplied to the engine set to a stoichiometric value, the vehicle exhibits brake specific fuel consumption (BSFC) characteristics as shown in FIG. 27A. In the figure, the abscissa represents the engine rotational speed NE and the ordinate the engine output (ps; metric horsepower). L1 to L3 each designate a curve along which the fuel consumption rate is constant. For example, on a curve L2, ,the fuel consumption rate is 220 g/psh. g/psh represents a unit of the fuel consumption rate, i.e. an amount of fuel consumption (grams) per ps and hour. As is clear from the figure, as the engine rotational speed NE and the engine output come nearer to the center of the characteristics diagram, the fuel economy is improved.
When the assistance of the traction motor 3 is not provided and the engine operating conditions correspond to a point A1 (NE=1500 rpm, engine output=10 ps) on the curve L2, the fuel consumption per hour is 220 g/psh.times.10 ps=2200 g/h. If the assistance of the traction motor 3 by 3.7 kilowatts is provided, the engine demanded output becomes equal to 5 ps, so that the operating point of the engine 1 in the figure moves to a point A2 on the curve L3. In this state, the fuel consumption per hour is 300 g/psh.times.5 ps=1500 g/h, which means that the fuel consumption is reduced by 700 g/h compared with the case of no assistance of the traction motor 3 being provided. However, the efficiency of the engine (fuel consumption rate) is degraded from 220 g/psh to 300 g/psh.
Further, when the engine operating condition without the assistance of the traction motor 3 corresponds to a point B1 (NE=3500 rpm, engine output=47 ps), the fuel consumption per hour is 195 g/psh.times.47 ps=9165 g/h. If the assistance of the traction motor 3 by 16 kilowatts is provided, the operating point of the engine 1 in the figure moves to a point B2 on the curve L2, which means that the fuel consumption per hour is 220 g/psh.times.25 ps=5500 g/h. Therefore, the fuel consumption per hour is reduced by 3665 g/h, but the efficiency of the engine (fuel consumption rate) is degraded from 195 g/psh to 220 g/psh.
As can be seen from the above, the conventional technique of assisting the engine by the traction motor can reduce the fuel consumption, but there remains room for improvement in respect of the efficiency of the engine (fuel consumption rate)